This invention relates to improvements in a trigger pulse forming circuit which forms a trigger pulse synchronous with an input pulse signal.
Heretofore, a differentiation circuit which consists only of a resistor and a capacitor has been known as a circuit which forms a trigger pulse for triggering a monostable multivibrator circuit or any other digital circuit. The differentiation circuit, however, has the disadvantage that trigger pulses whose pulse widths and amplitudes are stable cannot be formed on account of the rise and fall times of input pulse signals, the deviation of the time constant of the differentiation circuit, etc.
A circuit shown in FIG. 1 has therefore been proposed. The circuit of FIG. 1 includes a trigger pulse forming circuit, and a pulse shaping circuit which shapes an output from the trigger pulse forming circuit. The trigger pulse forming circuit comprises an emitter-coupled differential amplifier in which the emitters of transistors 1 and 2 are connected in common, the common emitters being grounded through a constant-current source 3. The collector of the transistor 1 is connected to a power supply +B.sub.2 through a load resistor 4, while the collector of the transistor 2 is connected to a power supply +B.sub.1 through an integration circuit composed of a load resistor 5 and a capacitor 6. Outputs from the transistors 1 and 2 are subjected to an AND operation by diodes 8 and 9. On the other hand, the pulse shaping circuit is constructed of a current switching circuit which comprises transistors 10 and 11 and a constant-current source 12. The voltage +V.sub.B1 of the power supply +B.sub.1 and the voltage +V.sub.B2 of the power supply +B.sub.2 are set in the relationship of V.sub.B1 &gt;V.sub.B2.
On the basis of an input pulse signal impressed on an input terminal IN, output pulses having phases opposite to each other are provided at the collectors of the transistors 1 and 2. The output pulse of the transistor 2 is integrated. The logical product between the integrated output and the output pulse from the transistor 1 is taken, to form a trigger pulse. Further, this pulse is waveform-shaped by the current switching circuit, to obtain the shaped trigger pulse. Numerals 7 and 15 designate power supplies for setting threshold levels.
In case of putting the trigger pulse forming circuit into an integrated circuit, it is desirable to make the electrostatic capacity of the capacitor 6 of the integration circuit very small. With the diminution of the electrostatic capacity of the capacitor 6, accordingly, it is required to make the resistance value of the load resistor 5 great and the current of the constant-current source 3 small. Consequently, the output impedance of the trigger pulse forming circuit becomes high, and the driving capability to drive a circuit at the succeeding stage such as pulse shaping circuit, level shifter and flip-flop is degraded. In other words, the trigger pulse forming circuit cannot form trigger pulses which are stable against the dispersions of the values of the circuit elements and the changes of the surrounding conditions including the drift, the change of the ambient temperature, etc. It is therefore problematic to fabricate the prior-art trigger pulse forming circuit in the form of an integrated circuit.